V₄C₃ MXene: a Type-II Nodal Line Semimetal with Potential as High-Performing Anode Material for Mg-Ion Battery

We have used density functional theory simulations to explore the topological characteristics of a new MXene-like material, V₄C₃, and its oxide counterpart, assessing their potential as anode materials for Mg-ion batteries. Our research reveals that V₄C₃ monolayer is a topological type-II nodal line semimetal, protected by time reversal and spatial inversion symmetries. This type-II nodal line is marked by unique drumhead-like edge states that appear either inside or outside the loop circle, contingent upon the edge ending. Intriguingly, even with an increase in metallicity due to oxygen functionalization, the topological features of V₄C₃ remain intact. Consequently, the monolayer V₄C₃ has a topologically enhanced electrical conductivity that amplifies further upon oxygen functionalization. During the charging phase, a remarkable storage concentration led to a peak specific capacity of 894.73 mAh g^-1 for V₄C₃, which only decreases to 789.33 mAh g^-1 for V₄C₃O₂. When compared to V₂C, V₄C₃ displays a significantly lower specific capacity loss due to functionalization, demonstrating its superior electrochemical properties. Additionally, V₄C₃ and V₄C₃O₂ exhibit moderate average open-circuit voltages (0.54 V for V₄C₃ and 0.58 V for V₄C₃O₂) and energy barriers for intercalation migration (ranging between 0.29-0.63 eV), which are desirable for anode materials. Thus, our simulation results support V₄C₃ potential as an efficient anode material for Mg-ion batteries.